Habitat conditions shape the phyllosphere mycobiome of the critically endangered Abies nebrodensis

The phyllosphere harbors diverse fungal communities that influence tree health, adaptation, and growth. This study investigated the phyllosphere mycobiome of the critically endangered Abies nebrodensis, using ITS2 metabarcoding. Samples were collected from 10 trees distributed across three sites during a single sampling campaign. Influence of tissue health state (green vs. blighted twigs) and variation of habitat conditions on diversity and composition of the mycobiome was evaluated. The 4, 501 amplicon sequence variants (ASVs) detected were dominated by Ascomycota, particularly Dothideomycetes and Sordariomycetes. The endospheric community showed reduced diversity compared to the full phyllospheric community; however, both were dominated by Valsaceae and Phaeosphaeriaceae and by the genera Phaeosphaeria and Lachnellula. Tissue health and site conditions significantly affected diversity and composition of the full phyllospheric but not of endospheric communities. Valsaceae were consistently detected within the core mycobiota of Sicilian fir, together with melanized and extremotolerant genera such as Knufia, Perusta, Cladosporium, Alternaria, and Sarcinomyces. Despite the study was based on a single spring snapshot, our findings indicate habitat variation as a major driver of the phyllosphere mycobiome of A. nebrodensis. The occurrence of Valsaceae and extremotolerant fungi in the core mycobiome suggests that these taxa may play a functional role potentially contributing to the holobiont adaptation to a harsh environment.